1. Field of the Invention
The present invention relates to a mixed conductor which exhibits both electron conduction and proton conduction. This mixed conductor can be used for the catalyst layer of a fuel cell, a gas diffusion catalyst and the like.
2. Description of the Related Art
The catalyst layer of a fuel cell is formed between a proton exchange membrane and a backing layer and supports a catalyst for accelerating an electrochemical reaction. The catalyst layer constitutes the electrodes of the fuel cell. In the catalyst layer on an air cathode side, for example, protons passing through the proton exchange membrane and electrons transferred to the air cathode are conducted to the catalyst, thus binding oxygen and protons diffused onto the catalyst. Thus, the catalyst layer needs to exhibit both proton conduction and electron conduction in order to reduce the transfer loss of oxygen, protons and electrons. To this end, a mixture of a poly electrolyte having catalysts supported on surfaces such as carbon particles (exhibiting electron conduction) and Nafion (trade name, manufactured by E.I du Pont de Nemours) which exhibits ionic conduction is used in the fuel cell.
However, if a material having ionic conduction and a material having electron conduction are used together, it is difficult to mix them completely uniformly. As a result, protons and electrons cannot be uniformly transferred to all catalyst particles.
To solve this disadvantage, there has been proposed a mixed conductor which exhibits both ionic conduction and electron conduction using one material.
For example, organic mixed conductors are disclosed in the following patent documents 1 to 4.
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-202971
Patent Document 2: Japanese Unexamined Patent Publication No. 2001-110428
Patent Document 3: Japanese Unexamined Patent Publication No. 2003-68321
Patent Document 4: Japanese Patent Application National Publication (Laid-Open) No. 2002-536787
In addition, inorganic mixed conductors conducting electrons and oxygen ions are disclosed in the following patent documents 5 to 8.
Patent Document 5: Japanese Unexamined Patent Publication No. (10)1998-255832
Patent Document 6: Japanese Unexamined Patent Publication No. (11)1999-335165
Patent Document 7: Japanese Unexamined Patent Publication No. 2000-251533
Patent Document 8: Japanese Unexamined Patent Publication No. 2000-18811
Since the organic mixed conductors are made of organic materials, they have many problems in terms of durability and heat resistance to be solved before being put to practical use.
Inorganic mixed conductors which transfer electrons and oxygen ions have high operating temperatures and therefore are inappropriate for small-sized fuel cells suitable for use in, for example, vehicles and cellular phones.